1. Technical Field of the Invention
The present invention relates to a magnetic bearing device.
2. Description of the Related Art
A magnetic bearing device has a rotating shaft which is a rotor, and magnetic poles which are provided at a stator to surround the rotating shaft and supports the rotating shaft in a non-contact manner with a magnetic force.
The rotating shaft is formed using a magnetic material, and is, for example, a rotating shaft of a turbo compressor, a cryogenic rotary machine, a turbo charger, a flywheel, or the like which rotates at high speed. A plurality of magnetic poles is arranged in the circumferential direction so as to surround the rotating shaft. In addition, in the present invention, the circumferential direction indicates the circumferential direction of the rotating shaft.
FIGS. 1A and 1B are configuration views of a homo-polar magnetic bearing device. FIG. 1B is a sectional view taken along the line B-B of FIG. 1A.
As illustrated in FIGS. 1A and 1B, a plurality of magnetic poles 15 is arranged in the circumferential direction so as to surround the rotating shaft 3. The rotating shaft 3 is floated and supported in a non-contact manner by passing a magnetic flux through the rotating shaft 3 which is being rotated at high speed from the magnetic poles 15 and by the magnetic attractive force thereof.
In the homo-polar magnetic bearing device, magnetic poles 15 of the same polarity (N-pole in FIG. 1A) are lined up in the circumferential direction of the rotating shaft 3 as illustrated in FIG. 1A, and an N-pole and an S-pole are lined up in the axial direction of the rotating shaft 3 as illustrated in FIG. 1B. As shown in FIG. 1A, adjacent magnetic poles 15 which are lined up in the circumferential direction are not close to each other. Thus, the intensity distribution of the magnetic field occurs between a portion with the magnetic poles 15 and a portion with no magnetic pole. Thus, an eddy current is generated so as to locally cancel out fluctuations in magnetic flux on the surface of the rotating shaft, in the circumferential direction. That is, an electromotive force which causes generation of an eddy current is shown by the following Formula (1). Even when magnetic flux density (magnetic field B) is small, a large eddy current will be generated when the rotating shaft rotates at high speed as in a high-speed rotating shaft.e∝B·v·L  (1)e: electromotive force, B: magnetic flux density, v: speed of traversing magnetic field, and L: length of conductor
FIGS. 2A and 2B and FIGS. 3A and 3B show the configuration of the magnetic bearing device of Patent Document 1. FIGS. 2B and 3B are sectional views taken along the lines B-B of FIGS. 2A and 3A, respectively. In Patent Document 1, in the homo-polar magnetic bearing device, an overhang portion 17 is provided, and N-poles 15 which are adjacent to each other in the circumferential direction are connected to each other in the circumferential direction (the case of FIGS. 2A and 2B), or brought close to each other (the case of FIGS. 3A and 3B). Thereby, the magnetic field flux density at an intermediate position between the magnetic poles 15 which are adjacent to each other in the circumferential direction are increased to reduce occurrence of the intensity distribution of the magnetic flux density in the circumferential direction. Thereby, generation of an eddy current is suppressed.
Moreover, in FIGS. 2A and 2B, and FIGS. 3A and 3B, the overhang portion 17 is thinly formed. Thus, magnetic saturation of the overhang portion 17 is caused, and thereby, the magnetic flux is prevented from leaking out from one of adjacent magnetic poles 15 to the other thereof.    Patent Document 1: Japanese Patent Application Laid-Open No. 2001-271836 (Magnetic Bearing Device)
However, the thickness of the overhang portion 17 of Patent Document 1 is, for example, about 0.3 to 1 mm, and the material thereof is laminated steel sheets. Therefore, there is a possibility that the strength drops, and work during manufacture becomes difficult. In practice, it is difficult to make all the thicknesses of all of the overhang portions 17 the same, and there is a possibility that the variations in the thickness of the overhang portion 17 may increase. That is, there is a possibility that variations may occur in the magnetic resistance of the overhang portion 17.